The medical tubes such as cannula, catheter etc. are generally inserted into blood vessels (particularly coronary artery), trachea, oviduct, urinary duct and various organs for the injection of liquids, patency of ducts and so on. These medical tubes are mainly made of TPU and the tip thereof is formed thin in a taper shape by hot drawing so as to facilitate insertion thereof into organs etc.
The method for forming a tube inclusive of such medical tube is exemplified by a method wherein TPU extruded from the delivery end of a die head is coated on a mandrel (hereinafter referred to as tubing method). A partial cross section of the die head to be used for the tubing method is shown in FIG. 3. In the tubing method, a mandrel 11 passing along an axial core 10a in a die head 10 is fed to the direction of an arrow b while a TPU 12 delivered from an extruder (not shown) is supplied into the die head 10, defined to a predetermined thickness and extruded from between a die 13 and a nipple 14, as shown in FIG. 3. Since the diameter of the TPU 12 at a delivery end 10b of the die head 10 is greater than that of the mandrel 11 by the thickness of the nipple 14, the TPU 12 falls to cover the mandrel 11.
In the tubing method, however, covering of mandrel 11 by TPU 12 under the open pressure is responsible for possible development of bubbles on the surface of the extruded product of TPU 12. For avoiding the development of the bubbles, the extrusion temperature of TPU 12 needs to be lowered, which in turn results in insufficient melting of TPU 12 to often cause occurrence of a granular structure (development of knob-like humps of from several dozens to several hundred .mu.m in diameter which appear on the surface as a result of the reaction in TPU to form a linear polymer having a higher molecular weight). Accordingly, it is not possible to prevent poor appearance due to the occurrence of bubbles or granular structure by the tubing method.
Another exemplification of the method for forming a tube is a method comprising extrusion-coating a mandrel with TPU in a die head (hereinafter referred to as pressurizing method). A partial cross section of the die head to be used for the pressurizing method is shown in FIG. 2. In the pressurizing method, a mandrel 2 passing along an axial core la in a die head 1 is delivered to the direction of an arrow a, while a TPU 3 delivered from an extruder (not shown) is supplied into the die head 1, and the TPU 3 is brought into contact with the mandrel 2 at a delivery end 4a of a nipple 4, as shown in FIG. 2. The mandrel 2 is further fed to the direction of the arrow a, whereby the TPU 2 is defined to a certain thickness by the mandrel 2 and a die 5 and the mandrel 2 covered with the TPU is extruded.
In the pressurizing method, the TPU 3 is coated on the mandrel 2 under pressurization, making a sharp contact with the aforementioned tubing method. Accordingly, the occurrence of bubbles or granular structure is reduced and the appearance of the tube becomes fine. On the other hand, hot drawability of the molding obtained by the pressurizing method is poor. That is, heating of the tip portion of the product to a temperature not lower than the glass transition temperature (Tg) by hot drawing to form same into a taper shape causes clouding of the drawn portion due to microcracks or occurrence of granular structure or even breakage of the portion.